Not Applicable.
Not Applicable.
1. Technical Field
The invention relates to a projection objective with a lens arrangement, which can be divided into six lens groups. The first, third, and sixth lens groups have positive power and the second and fourth lens groups respectively have negative power. The division of the lens system into lens groups is described in more detail hereinafter, based on the direction of propagation of the radiation.
The first lens group is positive and ends with a lens of positive power. A bulge is formed by the first lens group; it is unimportant if negative lenses are also arranged in the bulge.
The second lens group is of negative total power. This second lens group has as its first lens a lens having a concave lens surface toward the image. This second lens group substantially describes a waist. Here, also it is not of substantial importance if a few positive lenses are included in the second lens group, as long as the waist is maintained
The third lens group begins with a lens having positive power and a convex lens surface on the image side, and which can be a meniscus. If a thick meniscus lens is provided as the first lens, the separation of the lens groups can be considered to be within the lens.
The fourth lens group is of negative power. This fourth lens group begins with a lens of negative power, followed by several lenses having negative power A waist is formed by this lens group. It is unimportant if lenses having positive power are also contained within this lens group, as long as these influence the course of the beam over only a short distance and thus the waisted shape of the fourth lens group is maintained.
The fifth lens group has positive power overall. The first lens of this fifth lens group has a convex lens surface on the image side. A bulge is formed by the fifth lens group.
After the lens of maximum diameter (the bulge), there follow at least an additional two positive lenses in the fifth lens group, further negative lenses also being permitted.
The sixth lens group is likewise positive in its total power. The first lens of the sixth lens group is negative and has on the image side of concave lens surface. This first lens of the sixth lens group has a considerably smaller diameter in comparison with the maximum diameter of the bulge.
2. Background Art
Such projection objectives are in particular used in microlithography. They are known, for example, from the German Applications DE 198 55 108A, DE 198 55 157A, and DE 198 55 158A, in which the Applicant participated, and from the state of the art cited therein. These documents are incorporated herein by reference.
These projection objectives are usually constructed from purely spherical lenses, since the production and testing technology is advantageous for spheres.
Projection objectives are known from German Application DE 198 18 444 A1 which have lenses having aspheric surfaces in at least the fourth or fifth lens group. An increase of the numerical aperture and of the image quality can be attained by means of the aspheric surfaces. The projection objectives shown have a length from the mask plane to the image plane of 1,200 mm to 1,500 mm. A considerable use of material is associated with this length. High production costs are entailed by this use of material, since because of the required high image quality only high quality materials can be used. Aspheric lenses up to a diameter of about 300-mm are required, the provision of which is particularly expensive. It is not at all clear in the technical world whether aspheric lenses with such large lens diameters can be provided in the required quality. xe2x80x9cAspheric surfacesxe2x80x9d are understood to include all surfaces which are not spherical and which are rotationally symmetrical. Rotationally symmetrical splines can also be considered as aspheric lens surfaces.
The invention has as its object to provide a projection objective which has as few lenses as possible, with reduced use of material, the aspheric lens surfaces used being as few and as small as possible, with the lowest possible asphericity. A high aperture projection objective of short structure is to be cost-efficiently provided in this way.
The object of the invention is attained in particular by a projection objective for microlithography having a lens arrangement comprising a first lens group having positive power; a second lens group having negative power; a third lens group having positive power; a fourth lens group having negative power; a fifth lens group having positive power; and a sixth lens group having positive power; wherein a lens at the end of the second lens group, particularly the last lens of the second lens group, or a lens at the beginning of the third lens group, particularly the first lens of the third lens group, has an aspheric surface. In addition, the object of the invention is attained by a projection objective having a lens arrangement having at least a first waist of a pencil of rays, wherein the lens arrangement comprises at least one of the following a lens having an aspheric surface arranged before the first waist, a lens having an aspheric surface arranged after the first waist, and lenses having aspheric surfaces arranged before and after the first waist.
In a projection objective with a lens arrangement, by the measure of providing, in the forward half of this lens arrangement, at least one lens provided with an aspheric lens surface, the possibility was realized of furnishing a projection objective of compact construction and having a high image quality.
In the division of this lens arrangement into six lens groups: a first lens group having a positive power, a second lens group a negative power, a third lens group a positive power, a fourth lens group a negative power, and a fifth and sixth lens group respectively a positive power, a preferred position of the aspheric surface is at the end of the second lens group. It is then arranged, in particular, on the last lens of the second lens group or at the beginning of the third lens group, and indeed preferably on the first lens of the third lens group. A correction of image errors in the region between the image field zone and the image field edge is possible by means of this aspheric lens surface. In particular, the image errors of higher order, which become evident on considering sagittal sections, can be corrected. Since these image errors apparent in sagittal section are particularly difficult to correct, this is a particularly valuable contribution. In an advantageous embodiment, only one lens has an aspheric surface. This has a positive effect on the production costs, since it is the production of highly accurate aspheric surfaces that requires considerable technological effort, which entails increased costs. It was only with the use of exactly one aspheric lens that it was possible to provide a very compact objective, in which case the additional costs for the aspheric lens are not important, since considerable cost savings were connected with the reduction of the required material and of the surfaces to be processed and tested.
By the measure of providing lens arrangement that bas at least a first waist, an aspheric surface before and an aspheric surface after the waist, a lens arrangement is produced which makes possible a high numerical aperture with high image quality, particularly for the DUV region. In particular, it is possible by the use of these aspheric surfaces to furnish a projection objective of short structure and high image quality. Objectives used in microlithography generally have a high material density over their whole length, so that the reduction of the length is connected with a considerable saving of material. Since only very high-grade materials can be used for projection objectives, particularly for microlithography, the required use of material has a severe effect on the production costs.
The aspheric surface arranged before the first waist can be arranged at the end of the first lens group or at the beginning of the second lens group. Furthermore, it has been found to be advantageous to arrange an aspheric surface, arranged after the first waist, on the last lens of the second lens group or on the first lens of the third lens group.
The aspheric surface provided before the first waist in particular makes possible a targeted correction of coma in the region of the image field zone. This aspheric lens surface has only a slight effect on the skew spherical aberration in tangential section and in sagittal section. In contrast to this, the skew sagittal aberration, particularly in the region between the image field zone and image field edge, can be corrected by the aspheric lens surface after the waist.
The provision of a second aspheric lens surface is thus a worthwhile measure, in order to counter at high numerical aperture a reduction of image quality due to coma
In a few cases of application, particularly with very high numerical aperture, it has been found to be favorable to provide a projection objective wherein the third lens group has a lens having an aspheric surface, and, in particular, the last lens of the third lens group has an aspheric surface.
It has been found to be advantageous to provide a first lens in the sixth lens group with an aspheric surface for a further correction of coma, especially in the region of the image filed edge. For this aspheric lens surface the first lens of the sixth lens group has been found to be a particularly well suited position.
Furthermore, the numerical aperture can be increased, at constant image quality, by the provision of a further aspheric surface on the last lens of the third lens group.
It is an advantage of the invention to provide a refractive microlithographic projection objective, wherein all aspheric lens surfaces have a vertex radius (R) of at least 300-mm. Thus the aspheric surfaces are provided on long radii, since the production and testing is easier for lens surfaces with long radii. These surfaces are easily accessible to processing equipment because of their low curvature. In particular, surfaces with long radii are accessible with Cartesian coordinates for tactile measurement processes.
It bas been found to be advantageous to use at least two different materials for achromatization, for projection objectives designed for an illumination wavelength of less than 200 nm, because of the stronger dispersion of the lenses, even with the use of narrow-band light sources. In particular, fluorides, especially CaF2, are known as suitable materials, besides quartz glass.
It has been found to be advantageous to provide at least two leases of CaF2, which are arranged before an aperture stop in the fifth lens group, for the correction of color transverse errors.
It has been found to be advantageous for the fourth correction of color errors to integrate an achromat after the aperture stop by means of a positive CaF2 lens and a following negative quartz lens. This arrangement has a favorable effect on the correction of the spherical portions. In particular, longitudinal color errors can be corrected by the lenses after the aperture stop.
A reduction of the longitudinal error already results in general from the shortening of the length of the projection objective. Thus a good achromatization with a reduced use of CaF2 lenses can be attained with the objective according to the invention.